Interpretive Summary: Bacteria that cause disease in humans and animals are becoming increasingly resistant to antibiotics. This has caused a public health crisis because diseases once thought eradicated are reappearing. Bacteria are not only becoming resistant to one or two antibiotics, but they are also becoming resistant to many antibiotics; such bacteria are called multidrug-resistant bacteria. Swine and cattle producers have long been under pressure to limit the presence of disease-causing bacteria (pathogens) that are often present on retail meat products. However, with the emergence of multidrug-resistant pathogens, there are new pressures to limit the use of antimicrobial agents. However, there is evidence that the reduction in antibiotic use alone is not sufficient to reduce antimicrobial-resistant bacterial populations. The limitation of antimicrobial use poses an additional dilemma for producers with regard to the maintenance of healthy herds as well as preventing the presence of pathogens on retail meat. Many of the bacteria that are resistant to multiple drugs carry resistance genes on genetic elements called plasmids. Bacteria can carry multiple plasmids, but it may have a significant metabolic cost. Because of this cost, plasmids may be easily lost from the cells when antibiotics are not present. This study investigated the effect of Aureomycin, an antimicrobial given to swine, on Escherichia coli with differing numbers of plasmids when they were grown in mixed bacterial cultures swine fecal fluid. The results of this study showed that some plasmids were lost in some E. coli isolates, but plasmid loss did not correlate to Aureomycin treatment. Two strains had very stable plasmid content while two strains lost some but not all of their plasmids.

Technical Abstract:
The aim of this study was to determine differences in competitive fitness among E. coli strains with multiple plasmids when grown in commensal fecal bacteria from growing swine fed Aureomycin®–supplemented or –unsupplemented diets. Four multidrug-resistant (MDR) E. coli strains that possessed 2, 6, or 8 plasmids and one plasmid–free strain were inoculated into anoxic fecal cultures from swine fed a control (unsupplemented) or Aureomycin® (50g/ton) diet. On d 21 of Aureomycin® supplementation, fecal growth competition studies were performed. MDR E. coli were enumerated at 0, 6, and 24 h. The plasmid-free strain was eliminated from both groups by 24 hours. For each plasmid-bearing strain, there was no statistically significant difference in population (p < 0.05) between group treatments. There was no statistically significant difference in populations (p < 0.05) among all four plasmid–bearing strains when compared between treatments. These results suggest that the number and cost of maintenance of plasmids these MDR E. coli strains possessed had little influence on survivability in the short-term competitions studies employed. Additionally, the growth promotion level of Aureomycin® used in this study did not provide a growth advantage for the MDR strains.